Downhole ball and seat valves are utilized in a variety of applications in the production of hydrocarbons from subsurface wells. A common application is with downhole positive displacement pumps, where the pump has a standing valve and a traveling valve. However, in addition to use as standing valves and traveling valves in downhole pumps, downhole ball and seat valves have other application in hydrocarbon well applications such as operation of various downhole tools, including packers, washers and drill stem test tools.
Using downhole pumps as a specific application, these types of pumps typically utilize a standing valve and a traveling valve. In the general application, the standing valve is utilized with the pump barrel to allow fluid to flow into the barrel during an upstroke where the plunger is pulled upward through the barrel, and to prevent fluid from being displaced from the barrel when the plunger descends through the barrel. The traveling valve is utilized with the plunger, where the valve opens on the downstroke as the plunger descends through the fluid in the barrel and closes on the upstroke as the plunger rises lifting the fluid.
Ball and seat valves have the following general structure: (1) a cylindrical tubular cage forming a through-bore; (2) an annular seat extending perpendicularly across the bore, typically at its lower end; (3) a ball positioned within the bore of the cage; (4) a transverse ball stop extending across the upper end of the bore to limit the travel of the ball; and (5) a cylindrical shell which houses the cage, seat, ball, and ball stop. In the case of downhole pumps, fluid exits upwardly around the ball and through the upper end of the bore and out of the shell into the tubing.
Some downhole valves, particularly in the case of downhole pumps, may be subjected to continuous operation with the opening/closing sequence occurring thousands of times in a single day, frequently in a corrosive environment, pumping a fluid which may include abrasive solids. This repeated cycle naturally results in wear and tear. The seal formed between the ball and seat must be capable of withstanding substantial differential pressures without leaking.
It is known to provide “inserts” inside the tubular cage to increase the durability of the ball and seat valve and improve the volumetric flow and pressure drop characteristics through the pump. However, the known cage inserts have some disadvantages. A first common disadvantage is the presence of openings in the wall of the insert allow fluid to enter between the inside wall of the shell and the outside of the insert, trapping the fluid and providing continuous contact between the fluid and the pump components, promoting corrosion and erosion. As another disadvantage, an insert with openings requires a fluid seal at or below the ball seat. This fluid seal is typically an elastomer, which have the tendency to undergo compression set and wash out during lengthy periods of operation.
Another disadvantage of the commonly utilized cage inserts is the design of the ball stops at the top of the tubular cage. The commonly used ball stops are structural members which span across the opening at the top of the tubular cage. These structural members impede fluid flow through the cage.